The present invention relates to a guided-wave optical multi/demultiplexer which operates to multiplex and demultiplex two wave bands of light utilizing the principle of a Mach-Zehnder interferometer, and, more particularly, to a guided-wave optical multi/demultiplexer which is capable of operation in a wide wave band freely, and a method of multiplexing and demultiplexing.
A guided-wave optical multi/demultiplexer of the Mach-Zehnder interferometer type (hereinafter referred to as an "optical multi/demultiplexer") utilizing the principle of a Mach-Zehnder interferometer, has been described in, for example, an article entitled "Guided-Wave Optical WDM Circuit with Mach-Zehnder Interferometer configuration" by T. Kominato et al. in the Journal of Institute of Electronics, Information and Communication Engineers of Japan, C-I, Vol. J73-C-1, No. 5, pages 354 to 359, issued May, 1990. This publication discloses example which is constructed as shown in FIG.4. That is, this optical multi/demultiplexer is composed of two directional couplers 16, 17 having a coupling ratio of light intensity of k(.lambda.) and a phase shift region formed of two single mode guide-wave paths having guide-wave path lengths of L and L+.DELTA.L. When light having a wavelength of .lambda..sub.1 and light having a wavelength of .lambda..sub.2 are input at a port 20 at one end of a guide-wave path 14, light having a wavelength of .lambda..sub.1 is output from a port 21 at the other end of a guide-wave path 14, and the light having a wavelength of .lambda..sub.2 is output from a port 22 at the other end of a guide-wave path 13.
The condition of the phase shift region required for multiplexing and demultiplexing the light having a wavelength of .lambda..sub.1 and the light having a wavelength of .lambda..sub.2 is given by the following equation. (1) Providing that the equivalent refractive index of the guide-wave path is n(.lambda.), the difference in the optical path length, taking the equivalent refractive index into consideration, is n(.lambda.).DELTA.L, and N is an arbitrary integer. EQU n(.lambda.).DELTA.L=(N.+-.1/2).lambda..sub.1 =N.lambda..sub.2( 1)
It can be understood from equation (1) that the wavelengths .lambda..sub.1 and .lambda..sub.2 are not be arbitrarily obtained, but are restricted to a combination satisfying the following equation. EQU {.lambda..sub.1 i/n(.lambda..sub.1)}/.vertline..lambda..sub.1 /n(.lambda..sub.1)-.lambda..sub.2 /n(.lambda..sub.2) .vertline.=2N(2)
In Equation (2), the wavelength dependence of the equivalent refractive index of the guide-wave path is taken into consideration.
Equation (2) can be rewritten as follows, and the pass wavelength and the stop wavelength at each port can be accurately set by determining .DELTA.L in the following equation (3). EQU .DELTA.L=(N.+-.1/2).lambda..sub.1 /n(.lambda..sub.1)=N.lambda..sub.2 /n(.lambda..sub.2) (3)
In this optical multi/demultiplexer, letting the transmittance from port.sub.0 to port.sub.1 be T.sub.0-1, the transmittance from port.sub.0 to port.sub.2 be T.sub.0-2, each of the transmittances is given by each of the following equations. ##EQU1## Therein, when the guide-wave path satisfies the condition that exp(-.alpha..DELTA.L) is nearly equal to 1, the transmittances of port.sub.1 and port.sub.2 for each of the wavelengths .lambda..sub.1 and .lambda..sub.2 are derived from Equation (1) to Equation (5) and are given by the following equations. EQU T.sub.0-1 (.lambda..sub.1)=exp(-2.alpha..DELTA.L) (6) EQU T.sub.0-1 (.lambda..sub.2)={1-2k(.lambda..sub.2)}.sup.2 exp(-2.alpha..DELTA.L) (7) EQU T.sub.0-2 (.lambda..sub.2)=4k(.lambda..sub.2){1-2k(.lambda..sub.2)}(-2.alpha..DELTA. L) (8)) EQU T.sub.0-2 (.lambda..sub.1)=0 (9)
Taking notice of k(.lambda.) in Equation (6) to Equation (9), it can be understood that an optical multi/demultiplexer having a low insertion loss and having a cross-talk of 0 (zero) can be attained by setting the coupling ratio k(.lambda..sub.2) of the light intensity of wavelength .lambda..sub.2 for the light to be output from the port.sub.2.
FIGS. 5A and 5B are graphs showing the characteristic of an optical multi/demultiplexer designed under the condition of the wavelength .lambda..sub.1 =1.337 .mu.m and the wavelength .lambda..sub.2 =1.56 .mu.m according to this design method. FIG. 5A shows the insertion loss versus wavelength characteristic of the port.sub.1 21 when light is input at the port.sub.0 20, and FIG. 5B shows the insertion loss versus wavelength characteristic of the port.sub.2 22 when light is input at the port.sub.0 20. The pass wavelength band P whose loss is less than 0.1 dB in the port.sub.1 21 is a wavelength of 1.315 .mu.m to a wavelength of 1.350 .mu.m, and the band width is 0.034 .mu.m, which is narrow. On the other hand, the stop wavelength band C whose loss is more than 20 dB in the port.sub.2 22 is a wavelength of 1.322 .mu.m to a wavelength of 1.350 .mu.m, and the band width is 0.028 .mu.m, which is narrow. Low insertion loss and high isolation can be realized by utilizing a wavelength band restricted by both of the above wavelength bands for the wavelength band containing wavelength .lambda..sub.1. Since the pass wavelength band and the stop wavelength band in the port.sub.1 and the port.sub.2 are complementary, light in the band containing wavelength .lambda. .sub.1 and light in the band containing wavelength .lambda..sub.2 can be multiplexed and demultiplexed with low insertion loss and low cross-talk.
In the above conventional technology, however the wavelengths .lambda..sub.1, .lambda..sub.2 are not arbitrarily obtained but are limited to only the combination of wavelengths satisfying Equation (2). Therefore, the wavelength may not be freely chosen. In addition to this, there is a problem in that allowable range for wavelength shift due to deviation of the wavelength in the light source is narrow, since the width of the pass wavelength band and the width of the stop wavelength band are narrow.